To judge
the usefulness of a work, you should know the difference between primary
and secondary sources:

Primary
Source: A work that is itself the subject of your essay or (if you
are writing a historical research essay) a work written during the period
that you are writing about that gives you direct or primary knowledge of
that period.

Examples: the Constitution,
legal documents, memoirs, diaries, a work of literature, etc.

Secondary
Source:
Any commentary written both after and about the primary
source is a secondary source.

Examples: history textbook,
Times book review, encyclopedia reference, etc .